The main difference is that the new style allow one to adjust the rake of the rudder, check out the adjuster plate on the top of the upper casting/arm. Also there is a set screw at the bottom inside of the lower casting.
Over the years, our Club has had to TIG weld ALL old style castings on our four H-18's due to similar problems. They are all doing fine. Find a good MIG/TIG welder, show him both the 'good' and the 'broken' castings. You'll be okay, not cosmetically pretty, but functional.
1989 Hobie SX18 Sail # 1947 "In Theory..."
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'